JP2000134916A - Switching power circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power circuit capable of operating, even when a source voltage such as a battery voltage, etc., is lower, and being a small circuit scale. SOLUTION: This is a power circuit which has, in a route from a battery voltage VB up to a terminal J OUT for supplying an operating voltage VO to a CPU, a driving circuit 16 which turns on/off a P-channel MOSFET 7 whose source and drain are connected in series according to a driving signal, a smoothing circuit 15 which smoothes the drain voltage of the FBT 7 and outputs it to the above-mentioned terminal J OUT, and a driving signal outputting circuit 30 which outputs a driving signal to the driving circuit 16 so that the voltage VO of the terminal J OUT may be a set voltage. The driving circuit 16 has two transistors 25, 27 which connect a negative-side charge pump circuit 23 which generates a negative voltage, and the gate of the FET 7 to the output terminal J PO of the negative-side charge pump curcuit 23 and a ground voltage through two diodes 19, 21 respectively, as a turn-on driving unit which turns the FET 7 on.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply circuit for converting a power supply voltage into a predetermined set voltage and supplying the same to a power supply target such as a microcomputer.

[0002]

2. Description of the Related Art In recent years, the operable voltage of a microcomputer (hereinafter referred to as a CPU) mounted on an electronic control unit or the like tends to decrease from about 5 V to 3.3 V or 2.5 V.

For this reason, it is desired that a power supply circuit for supplying an operating voltage to a CPU can stably supply an operating voltage even when a power supply voltage (such as a battery voltage) as a power source is low. I have. In addition, since the current consumption of the CPU tends to increase with an increase in the frequency of the operation clock, the type of the power supply circuit is a switching power supply circuit (switching regulator) instead of a series power supply circuit (series regulator). ) Is becoming mainstream.

That is, in a series type power supply circuit, a power supply transistor provided in series in a power supply path from a power supply voltage to a power supply target is linearly controlled so that a voltage supplied to the power supply target becomes a predetermined voltage. Loss due to the power supply transistor (power consumption)
Becomes large.

On the other hand, a switching power supply circuit is
Generally, a power supply transistor provided in series in a power supply path from a power supply voltage to a power supply target, a switching drive circuit for turning on / off the power supply transistor in accordance with a drive signal, and a power supply transistor A smoothing circuit that is provided on a path between the power supply target and a coil or a capacitor that smoothes the output voltage of the power supply transistor and supplies the output voltage to the power supply target, and the voltage smoothed by the smoothing circuit is predetermined. And a drive signal output circuit that outputs a drive signal to the switching drive circuit so that the set voltage is obtained. And
In this type of switching power supply circuit, a predetermined operating voltage is supplied to the power supply target by repeatedly turning on / off the power supply transistor. Heat loss of the transistor can be suppressed.

[0006]

Here, in the above-mentioned switching power supply circuit, when it is intended to operate even when the power supply voltage is low, a PNP-type bipolar transistor (PNP transistor) is used as a power supply transistor. It is conceivable to use.

However, the bipolar transistor has a drawback that the transient loss at the time of the switching operation is large, and the bipolar transistor capable of flowing a large current cannot perform the switching operation at a very high frequency. Therefore, a field effect transistor (hereinafter, referred to as an FET) is effective as a power supply transistor.

However, if a P-channel FET is simply used as a power supply transistor instead of a PNP transistor, a sufficient gate-source voltage of the FET cannot be secured when the power supply voltage is low.
Either the operation stops, or the on-resistance of the FET becomes extremely large.

That is, in this case, the P-channel FET is
The source is connected to the power supply voltage side in the power supply path,
The drain is connected to the smoothing circuit side. Then, when the power supply voltage becomes lower than the gate-source threshold voltage VGS (Pth) at which the P-channel FET can be turned on, the gate voltage of the P-channel FET is reduced to the ground voltage (0 V) by the switching drive circuit. Even if it is lowered, it will not turn on.

Further, as a power supply transistor, N
When a channel FET is used, its drain is connected to the power supply voltage side of the power supply path, and its source is connected to the smoothing circuit side. If it is necessary to supply power, especially if it is to be operated even when the power supply voltage is low, the number of connection stages such as capacitors and inverters that compose the charge pump circuit for boosting the power supply voltage increases, resulting in an increase in circuit size and cost. Becomes noticeable.

For example, a threshold voltage VGS (Nth) between the gate and the source at which the N-channel FET can be turned on is 5 V,
Assuming that the power supply voltage is 3 V, in order to turn on the N-channel FET, a voltage (= 8 V = 3 V + 5 V) higher than the power supply voltage by at least the gate-source threshold voltage VGS (Nth) is applied to the gate. Must be generated and given.

The present invention has been made in view of such a problem, and has as its object to provide a small-scale switching power supply circuit that can operate even when the power supply voltage is lower.

[0013]

Means for Solving the Problems and Effects of the Invention In order to achieve the above object, in the switching power supply circuit of the present invention according to the first aspect, the switching drive circuit changes the power supply voltage to the power supply target. A power supply transistor provided in series in a power supply path leading to the power supply transistor is turned on / off (switching operation) in response to a drive signal from a drive signal output circuit, and the power supply transistor is connected between the power supply transistor and a power supply target. A smoothing circuit provided in the path smoothes the output voltage of the power supply transistor (that is, a pulse-like voltage accompanying the switching operation of the power supply transistor) and supplies the output voltage to the power supply target. Then, the drive signal output circuit outputs a drive signal to the switching drive circuit such that the voltage smoothed by the smoothing circuit and supplied to the power supply target becomes a predetermined set voltage.

In particular, in the switching power supply circuit of the present invention, a P-channel F
An ET (P-channel field-effect transistor) is used. The P-channel FET has a source connected to the power supply voltage side and a drain connected to the smoothing circuit side in the power supply path.

Further, the switching drive circuit serves as an ON drive section for turning on the P-channel FET, receives a power supply voltage, generates a negative voltage lower than the ground voltage, and outputs the negative voltage from an output terminal. A negative-side charge pump circuit; and a switching element for connecting a gate of the P-channel FET to an output terminal of the negative-side charge pump circuit in response to a drive signal from a drive signal output circuit.

That is, a negative voltage lower than the ground voltage (= 0 V) is generated by the negative charge pump circuit, and the gate of the P-channel FET as a power supply transistor is connected to the negative charge pump circuit via the switching element. The P-channel FET is turned on by connecting to the negative voltage generated by

Therefore, according to the switching power supply circuit of the present invention, even if the power supply voltage becomes lower than the gate-source threshold voltage VGS (Pth) at which the P-channel FET can be turned on, the P-channel FET can be turned on. The P-channel FET can be reliably turned on by sufficiently securing the gate-source voltage, and can operate even when the power supply voltage is lower.

In particular, in the switching power supply circuit of the present invention, a P-channel FE is used as a power supply transistor.
Since T is used, the negative-side charge pump circuit supplies a negative-side voltage smaller than the value of the gate-source threshold voltage VGS (Pth) that can be turned on of the P-channel FET with reference to the ground voltage. Since it is sufficient to generate the negative charge pump circuit, the number of connection stages such as capacitors and inverters constituting the negative side charge pump circuit can be reduced, and the circuit size and cost can be minimized.

That is, as described above, when an N-channel FET is used as a power supply transistor, the gate-source threshold voltage VGS ( Nth), and generates a voltage boosted to the positive side by the amount of
However, according to the switching power supply circuit of the present invention, assuming that the minimum value of the operable power supply voltage is Vmin (<VGS (Pth)), the gate power supply is controlled with respect to the ground voltage. The difference between the source-to-source threshold voltage VGS (Pth) and the minimum value Vmin of the power supply voltage (= VGS (Pth) −V
min) to generate a voltage boosted to the negative side, and
If the power is supplied to the gate of the channel FET, the P-channel FET as the power supply transistor can be reliably turned on, and the amount of boost from the ground voltage to the negative side can be reduced.

To be more specific, for example, a gate-source threshold voltage VGS (Pth) that can turn on a P-channel FET and a gate-source threshold voltage VGS that can turn on an N-channel FET will be described. (Nth) and both are 5V
If it is assumed that the operation is performed even when the power supply voltage is 3 V, the N-channel FET does not turn on unless a voltage (= 8 V) boosted by 5 V with respect to the power supply voltage is applied to the gate. On the other hand, in the case of the P-channel FET, the difference between the gate-source threshold voltage VGS (Pth) and the power supply voltage (2V = 5) with reference to the ground voltage (= 0).
V-3V), the gate can be turned on by applying a voltage of -2V raised to the negative side to the gate.

Therefore, according to the switching power supply circuit of the present invention, it is possible to operate even when the power supply voltage is lower despite the small-scale circuit configuration. Next, in the switching power supply circuit according to the second aspect, the ON drive section of the switching drive circuit connects the gate of the P-channel FET to the ground voltage according to the drive signal from the drive signal output circuit. Device. According to this switching power supply circuit, the turn-on time (time from the OFF state to the ON state) of the P-channel FET can be shortened even if the current-drawing ability of the negative-side charge pump circuit is small. And P channel FE
The switching speed of T can be improved.

That is, in the switching power supply circuit according to claim 1, when the P-channel FET is turned on from the off state, the switching element of the on-drive unit is turned on, and the gate of the P-channel FET is connected to the negative side charge pump. By drawing the current (charge) to the output terminal side of the circuit, the gate-source capacitance CGS of the P-channel FET is charged. In this case, if the current drawing ability of the negative side charge pump circuit is small, the gate will -The source-to-source capacitance CGS cannot be charged sufficiently and quickly, and the turn-on time of the P-channel FET becomes long.

On the other hand, as described in claim 2, the second
2. When the P-channel FET is turned on from the off state, the second switching element and the switching element according to claim 1 (hereinafter, referred to as the first switching element) are both turned on. Until the gate voltage of the P-channel FET reaches a predetermined voltage near the ground voltage, the gate-source capacitance CGS becomes the second voltage.
Rapidly charged through the switching element of
The charging current of the gate-source capacitance CGS flows from the gate of the P-channel FET to the ground voltage via the second switching element), and after the gate voltage reaches the predetermined voltage, the gate-source capacitance CGS becomes the second voltage. The charging is performed via the first switching element (that is, the charging current flows from the gate of the P-channel FET to the output terminal of the negative-side charge pump circuit via the first switching element). As described above, since the gate-source capacitance CGS of the P-channel FET can be rapidly charged via the second switching element, even if the current drawing ability of the negative-side charge pump circuit is small, the P-channel FET is not charged. The turn-on time can be shortened.

By the way, in the switching power supply circuit according to claim 2, as the second switching element,
An NPN-type bipolar transistor (NPN transistor), an N-channel FET, an analog switch, or the like can be used. This is the same for the first switching element.

For example, when an NPN transistor is used as the second switching element, its collector is connected to the gate of the P-channel FET, and its emitter is connected to the ground voltage. When an N-channel FET is used as a switching element, its drain is connected to the gate of the P-channel FET, and its source is connected to the ground voltage.

However, as the second switching element, an NPN transistor having a forward parasitic diode between the emitter and the collector and a forward parasitic diode between the source and the drain are provided. N channel F
When ET is used, the following problem occurs.

That is, as described above, when turning on the P-channel FET as the power supply transistor,
The first switching element and the second switching element are both turned on, and the gate voltage of the P-channel FET finally tries to become a negative voltage by the negative side charge pump circuit via the first switching element. When the second switching element has a parasitic diode as described above,
A sneak current flows from the ground voltage to the output terminal of the negative side charge pump circuit via the parasitic diode of the second switching element and the first switching element,
As a result, the gate voltage of the P-channel FET drops only to a voltage (-VF) lower than the ground voltage by a forward drop voltage VF (about 0.6 V) of the parasitic diode. Moreover, since an extra current flows from the ground voltage to the output terminal of the negative charge pump circuit via the parasitic diode of the second switching element and the first switching element, the output of the negative voltage in the negative charge pump circuit Ability will be reduced. Such a problem also applies to a case where an analog switch is used as the second switching element.

Therefore, in the switching power supply circuit according to the second aspect, the P-channel F
If a diode is provided in the path between the gate of the ET and the second switching element in the forward direction, the first switching element may be used regardless of the configuration of the second switching element. When both the switching element and the second switching element are turned on, the second
It is possible to reliably prevent current from flowing to the output terminal of the negative side charge pump circuit via the switching element and the first switching element, and to solve the above problem.

Incidentally, such a sneak current can be prevented basically by providing a diode only in the path between the gate of the P-channel FET and the second switching element. Only a diode is provided in order to turn on the P-channel FET.
When the first switching element and the second switching element are turned on, the second switching element cannot sufficiently draw current from the gate of the P-channel FET due to the effect of the forward drop voltage of the added diode. there is a possibility. Therefore, in the switching power supply circuit according to the third aspect, a diode is also provided in a path between the gate of the P-channel FET and the first switching element, so that the path on the second switching element side and the first switching element are connected to each other. The operation and effects similar to those of the switching power supply circuit according to the second aspect can be further ensured by balancing the path with the switching element. That is, the P channel FE
The gate voltage of T is approximately the forward drop voltage VF of the diode
Until the P-channel FET reaches the ground voltage through the second switching element from the gate of the P-channel FET
The charging current of the gate-source capacitance CGS flows, and the gate-source capacitance CGS is rapidly charged. As a result, even if the current drawing capability of the negative side charge pump circuit is small, the turn-on time of the P-channel FET is small. Can be shortened.

On the other hand, the negative-side charge pump circuit includes an oscillating circuit that receives a power supply voltage and outputs an oscillating signal of a predetermined frequency, and generates a negative voltage from the voltage of the oscillating signal output from the oscillating circuit. Although the negative voltage is sequentially accumulated and accumulated in the capacitors of a plurality of stages, radiation noise to the outside may be generated due to the oscillation signal for generating the negative voltage.

Therefore, in the switching power supply circuit according to the fourth aspect, in the switching power supply circuit according to the second or third aspect, the negative side charge pump circuit includes a power supply voltage having a predetermined threshold voltage VTH. In these cases,
It is configured to stop the operation of generating the negative voltage.

That is, the power supply voltage is higher than the gate-source threshold voltage VGS (Pth) at which the P-channel FET can be turned on, and the gate of the P-channel FET is connected to the ground voltage via the second switching element. Just P channel F
If the ET can be turned on, the negative charge pump circuit is unnecessary, and in such a state, the operation of the negative charge pump circuit is stopped. According to the switching power supply circuit of the fourth aspect, it is very advantageous to reduce radiation noise from the switching power supply circuit.

The threshold voltage VTH is higher than the gate-source threshold voltage VGS (Pth) of the P-channel FET by a margin α (= VGS (Pth) +
α). Next, in a switching power supply circuit according to a fifth aspect, in the switching power supply circuit according to the first to fourth aspects, at least a negative side charge pump circuit is formed on a semiconductor substrate as an IC, and the negative side charge pump circuit is connected to the negative side. Each element constituting the side charge pump circuit is separated by an insulator. That is, in the switching power supply circuit according to the fifth aspect, the negative side charge pump circuit is formed into an IC by an insulation separation step in which each element is separated by an insulator.

The reason will be described below. First, as described above, the negative-side charge pump circuit generates a negative voltage from the voltage of the oscillation signal output from the oscillation circuit,
The negative voltage is stored in a plurality of stages of capacitors. For example, as shown in FIG. 7 showing a cross-sectional structure of the capacitor formed in the junction separation step, a negative side charge pump circuit is formed in the junction separation step. Then, a parasitic diode Dk1 is generated between the P ⁺ part serving as isolation connected to the ground voltage and the N ⁺ part (emitter diffusion layer) connected to the negative aluminum electrode of the capacitor.

For this reason, the potential of the negative aluminum electrode of the capacitor is reduced only to -VF (where VF is the forward voltage drop of the parasitic diode Dk1).
The negative voltage required to turn on the channel FET cannot be generated.

On the other hand, if the negative-side charge pump circuit is formed in the insulation separation step, each element is separated by an insulator (so-called trench), and the parasitic diode as in the case of using the junction separation step. Since Dk1 is not automatically formed, a negative-side charge pump circuit capable of generating a desired negative voltage can be obtained.

Also, as described in claim 6, claim 1
In the switching power supply circuit according to any one of claims 1 to 4, when at least the switching drive circuit and the drive signal output circuit are formed on a semiconductor substrate as an IC, the switching drive circuit and the drive signal output circuit are each made of an insulator. If it is formed in the insulation separation process so that it is separated,
Similarly to the switching power supply circuit according to the fifth aspect, it is possible to obtain a negative charge pump circuit capable of generating a desired negative voltage, and a negative voltage is generated in each unit other than the negative charge pump circuit. Since an unnecessary parasitic diode is not formed in a portion to be switched, the switching power supply circuit can be surely formed into an IC.

[0038]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. First, FIG. 1 is a circuit diagram illustrating a configuration of a switching power supply circuit according to an embodiment to which the present invention is applied.

The switching power supply circuit according to the present embodiment is provided in an electronic control unit for controlling an engine, a transmission, and the like of a vehicle, and has a voltage at a positive terminal of a battery mounted on the vehicle (corresponding to a power supply voltage; VB) is converted to a predetermined set voltage, and the converted voltage is supplied to a CPU (microcomputer) (not shown) as a power supply target provided in the electronic control unit, for operating voltage VO. It is supplied as.

As shown in FIG. 1, the switching power supply circuit of the present embodiment includes a power supply input terminal JIN connected to a battery voltage VB as a power supply voltage, and a power supply input terminal JIN.
, One end is connected to the end of the coil 3 opposite to the battery voltage VB side, and the other end is connected to the ground voltage (0 V which is the voltage of the negative terminal of the battery). A capacitor 5, a P-channel MOSFET (P-channel MOS field effect transistor) 7 as a power supply transistor whose source is connected to the end of the coil 3 on the side opposite to the battery voltage VB side,
Flywheel diode 9 connected to the drain of channel MOSFET 7 and the anode connected to ground voltage
And a coil 11 having one end connected to the drain of the P-channel MOSFET 7 and the other end connected to a power output terminal JOUT for outputting the operating voltage VO to the CPU, and one end connected to the P-channel MOSFET 7 side of the coil 11. A capacitor 13 connected to the opposite end and the other end connected to the ground voltage.

That is, in the switching power supply circuit of this embodiment, the electric path from the power input terminal JIN to the power output terminal JOUT is a power supply path from the battery voltage VB to the CPU. , The source and the drain of the P-channel MOSFET 7 are connected in series with the source being on the battery voltage VB side. A path between the P-channel MOSFET 7 and the CPU (specifically, a path between the drain of the P-channel MOSFET 7 and the power supply output terminal JOUT) includes a P-channel MOSFET.
Smoothing the output voltage (drain voltage) of SFET7 to C
Coil 11 and capacitor 13 to supply to PU
Is provided.

In the switching power supply circuit of the present embodiment, the switching drive circuit 16 for turning on / off the P-channel MOSFET 7 according to a drive signal has an emitter connected to the power supply input terminal JIN, and a collector connected to the gate of the P-channel MOSFET 7. And two diodes 19 and 21 whose anodes are connected to the gate of the P-channel MOS FET 7 respectively.
And a negative-side charge pump circuit 23 that receives the battery voltage VB from the power supply input terminal JIN to generate a negative voltage lower than the ground voltage and outputs the negative voltage from the output terminal JPO (see FIG. 2). An NPN transistor 25 as a first switching element connected to the cathode of the diode 19 and having an emitter connected to the output terminal JPO of the negative side charge pump circuit 23, and a collector connected to the diode 21
And an NPN transistor 27 as a second switching element having an emitter connected to the ground voltage.

In this embodiment, two diodes 19 and 21, a negative-side charge pump circuit 23, and two NPN transistors 25 and 27 among the elements constituting the switching drive circuit 16 are connected to each other. Channel MOS
PN corresponds to an ON drive section for turning on the FET 7
P transistor 17 corresponds to an off drive unit for turning off P channel MOSFET 7.

Further, the switching power supply circuit of the present embodiment includes a smoothing circuit 15 (the coil 11 and the capacitor 1).
Operation voltage V smoothed by 3) and supplied to CPU
O (that is, the voltage of the power output terminal JOUT) is set to a predetermined set voltage.
A driving signal output circuit 30 for outputting a driving signal to the base of each of the transistors 17, 25, and 27, two voltage dividing resistors 31 and 33 provided in series between a power supply output terminal JOUT and a ground voltage; The two voltage dividing resistors 31 and 3
An error amplifier 35 that outputs a voltage signal proportional to the difference between the voltage at the connection point 3 and a predetermined reference voltage VREF, and a triangular wave generating circuit 37 that outputs a triangular wave having the same cycle as the switching cycle for turning on / off the P-channel MOSFET 7. When,
The triangular wave from the triangular wave generating circuit 37 and the voltage signal from the error amplifier 35 are compared in magnitude, and when the level of the triangular wave is equal to or higher than the voltage signal from the error amplifier 35, the high-level drive signal is sent to each of the transistors 17, 25, When the level of the triangular wave is lower than the voltage signal from the error amplifier 35, a low-level drive signal is output to the base of each of the transistors 17, 25, and 27.
And a WM comparator 39.

Next, as shown in FIG. 2, the negative charge pump circuit 23 includes two voltage dividing resistors 41 and 42 provided in series between the battery voltage VB and the ground voltage, and a constant voltage source ( For example, 5V) Two voltage dividing resistors 43 and 44 provided in series between Vcc and the ground voltage, and the voltage V1 at the connection point between the voltage dividing resistors 41 and 42 is equal to the voltage dividing resistors 43 and 44.
When the voltage is equal to or higher than the voltage V2 at the connection point 44, a high level signal is output, and the voltage V1 at the connection point between the voltage dividing resistors 41 and 42 is lower than the voltage V2 at the connection point between the voltage dividing resistors 43 and 44. In this case, a comparator 45 that outputs a low-level signal and an NPN transistor 46 whose base is connected to the output terminal of the comparator 45 and whose emitter is connected to the ground voltage are provided.

Further, the negative side charge pump circuit 23 has one end connected to the battery voltage VB and the other end connected to the collector of the NPN transistor 46.
And a collector and a base connected to the collector of the NPN transistor 46 and an emitter connected to the ground voltage.
A PN transistor 48, an NPN transistor 49 having a base connected to the collector and base of the NPN transistor 48, an emitter connected to the ground voltage, and forming a current mirror circuit with the NPN transistor 48;
A constant current source 50a having one end connected to the battery voltage VB and the other end connected to the collector of the NPN transistor 49;
And one end is connected to the battery voltage, and the constant current source 5
0a, a constant current source 50b for flowing the same current as the current flowing to 0a,
An oscillation circuit 52 provided between the constant current source 50b and the ground voltage for receiving an electric power supplied from the battery voltage VB via the constant current source 50b and outputting an oscillation signal of a predetermined frequency. ing.

Further, the negative-side charge pump circuit 23
Are a plurality of (two in this embodiment) inverters IN that sequentially invert and output the oscillation signal from the oscillation circuit 52.
V1, INV2 and their respective inverters INV1, INV
Capacitors C1, whose one ends are respectively connected to the output terminals of
C2, a diode D1 having a cathode connected to the ground voltage, an anode connected to the end of the capacitor C1 opposite to the inverter INV1 side, a cathode connected to the anode of the diode D1, and an anode connected to the capacitor C2.
A diode D2 connected to an end opposite to the inverter INV2 side, a diode 54 having a cathode connected to the anode of the diode D2, and an anode connected to the output terminal JPO of the negative side charge pump circuit 23; A capacitor 56 having one end connected to the output terminal JPO and the anode of the diode 54 and the other end connected to the ground voltage;
It has.

On the other hand, in the switching power supply circuit of the present embodiment, the circuit portion surrounded by the dashed line in FIG. 1 (that is, the switching drive circuit 16 and the drive signal output circuit 30).
As shown in FIG. 3, each element is formed into an IC by an insulation separation process in which each element is separated by an insulator called a trench.

FIG. 3 shows a cross-sectional structure of the capacitor formed in the insulation separation step. Then, in the IC formed in this insulation separation step, the outer periphery of each element is formed.
The Deep N ⁺ portion is connected to the ground voltage in order to stabilize the potential. However, since each element is insulated and separated from each other by a trench made of an insulator, the parasitic diode Dk1 as in the junction separation step shown in FIG. Is not automatically formed.

Next, the operation of the switching power supply circuit of the present embodiment configured as described above will be described. First, in the negative charge pump circuit 23 shown in FIG. 2, when the battery voltage VB is equal to or higher than a predetermined threshold voltage VTH, the voltage V1 at the connection point between the two voltage dividing resistors 41 and 42 becomes 2 V at the connection point between the two voltage dividing resistors 43 and 44
As a result, the comparator 45 outputs a high-level signal to the base of the NPN transistor 46. As a result, the NPN transistor 46 is turned on, and both of the two NPN transistors 48 and 49 forming the current mirror circuit are turned off. You.

Therefore, when the battery voltage VB is equal to or higher than the threshold voltage VTH, current does not flow to the constant current source 50a via the NPN transistor 49, so that current also flows from the constant current source 50b to the oscillation circuit 52. The flow stops, and as a result, the operation of the oscillation circuit 52 stops.

In the present embodiment, the threshold voltage VTH is equal to the gate-source threshold voltage VGS (Pth) (= 5 V in this embodiment) of the P-channel MOSFET 7 which can be turned on. Forward voltage drop VF (=
About 0.6V) and the collector-emitter voltage VCE (T27) (= about 0.1V) when the NPN transistor 27 is on.
And a predetermined margin, and is set to, for example, 6V. That is, when the battery voltage VB is 6 V or more, the comparator 45 sets the resistance values of the voltage dividing resistors 41 and 42 and the resistance values of the voltage dividing resistors 43 and 44 to High.
The level signal is set to be output.

On the other hand, in the negative charge pump circuit 23, when the battery voltage VB is lower than the threshold voltage VTH, the voltage V1 at the connection point between the two voltage dividing resistors 41 and 42 becomes two voltage dividing voltages. When the voltage becomes lower than the voltage V2 at the connection point between the resistors 43 and 44, the signal output from the comparator 45 to the base of the NPN transistor 46 becomes low level. As a result, the NPN transistor 46 is turned off and the two NPN transistors 48 , 49 operate as a current mirror circuit.

Therefore, when the battery voltage VB is lower than the threshold voltage VTH, a constant current flows to the constant current source 47 via the NPN transistor 48, and a constant current proportional to the constant current is NPN. Transistor 49
Flows through the constant current source 50a through the
0a is supplied from the constant current source 50b to the oscillation circuit 52, and the oscillation circuit 52
It receives an electric power supplied from the battery voltage VB via the constant current source 50b and outputs an oscillation signal of a predetermined frequency.

When an oscillation signal is output from the oscillation circuit 52, a negative voltage is generated from the voltage of the oscillation signal by the inverters INV1 and INV2, the capacitors C1 and C2, and the diodes D1 and D2. Finally, it is held by the capacitor 56 connected to the output terminal JPO.

More specifically, first, the oscillation circuit 52
Is low level (= 0V), the output of the inverter INV1 is high level (= VB),
When the output of NV2 is at the low level (= 0V), the capacitor C1 is charged, and the potential difference between both ends of the capacitor C1 becomes [VB-VF]. Where VF is the diode D1
And the general forward voltage drop of the diode (about 0.6 V).

Next, the oscillation signal from the oscillation circuit 52 is inverted to the high level (= VB), and the inverter INV
1 output is low level (= 0V), inverter INV2
Becomes high level (= VB), the capacitor C
1 is prevented by the diode D1, the voltage at the anode of the diode D1 and the diode D1
2 is reduced to [-VB + VF], the capacitor C2 is charged, and the capacitor C2 is charged.
Is [2.times.VB -2.times.VF].

Then, the oscillation signal from the oscillation circuit 52 is again inverted to the low level (= 0 V) and the inverter INV
1 is High level (= VB), the inverter INV2
When the output of the capacitor becomes low level (= 0V), the capacitor C
2 is prevented by the diode D2, the voltage at the anode of the diode D2 and the diode 5
The voltage of the cathode of No. 4 drops to [-2.times.VB + 2.times.VF], the capacitor 56 is charged by the negative voltage through the diode 54, and the voltage of the output terminal JPO becomes [-2.
.Times.VB + 3.times.VF].

In the above description, the capacitors C1, C2,
The negative voltage output from the output terminal JPO (the negative voltage stored in the capacitor 56) is actually a case where it is assumed that there is no charge delay or a charge loss at the time of charging of the capacitor 56.
The negative voltage is slightly higher than the above [−2 × VB + 3 × VF].

Further, in this embodiment, even when the battery voltage VB drops to 3 V, the negative voltage output from the output terminal JPO of the negative side charge pump circuit 23 becomes negative.
The voltage of the capacitors C1, C2, 56 and the capacitances of the capacitors C1, C2, Inverters INV1, IN
V2 and the number of connection stages of the diodes D1 and D2 are set. That is, in FIG. 2, the capacitors C1 and C2, the inverters INV1 and INV2, and the diodes D1 and D2
Have a two-stage configuration, but the number of connection stages of these elements can be set as appropriate.

Next, the operation of the entire switching power supply circuit shown in FIG. 1 will be described. First, in the switching power supply circuit of the present embodiment, as will be described in detail later, when a high-level drive signal is output from the PWM comparator 39 of the drive signal output circuit 30, the switching drive circuit 1
6, the P-channel MOSFET 7 is turned on. Conversely, when the PWM comparator 39 outputs a low-level drive signal, the switching drive circuit 16 turns the P-channel MOSFET 7 off.

When the P-channel MOSFET 7 is turned on / off, the P-channel MOSFET 7
A pulse-like voltage is output from the drain of the power supply terminal. The pulse-like voltage is smoothed by a smoothing circuit 15 (low-pass filter) including a coil 11 and a capacitor 13, and the smoothed voltage is supplied to a power supply output terminal. It is supplied as an operating voltage VO from JOUT to the CPU.

When the P-channel MOSFET 7 is turned off, a circulating current flows through the coil 11 by the flywheel diode 9. Also, the power input terminals JIN and P
Due to the low-pass filter including the coil 3 and the capacitor 5 provided between the P-channel MOSFET 7 and the channel MOSFET 7, noise generated due to the on / off operation (switching operation) of the P-channel MOSFET 7 is mixed into the battery voltage VB or becomes radio noise. Is prevented.

Here, the PWM comparator 39 of the drive signal output circuit 30 sends a pulse width modulation signal (hereinafter, referred to as a PWM signal) with one cycle of the triangular wave output from the triangular wave generation circuit 37 to the switching drive circuit 16. Is output as a drive signal. The duty ratio of the PWM signal (the High level time per one cycle of the PWM signal) is such that the voltage at the connection point between the voltage dividing resistors 31 and 33 becomes higher than the reference voltage VREF because the voltage at the power output terminal JOUT increases. Indeed, it gets smaller. That is, the voltage dividing resistor 3
As the voltage at the connection points 1 and 33 becomes higher than the reference voltage VREF, the level of the voltage signal output from the error amplifier 35 increases, and the level of the triangular wave output from the triangular wave generation circuit 37 changes. This is because the time when the voltage signal becomes higher than the voltage signal is shortened.

Conversely, the duty ratio of the PWM signal increases as the voltage at the power supply output terminal JOUT decreases and the voltage at the connection point of the voltage dividing resistors 31, 33 becomes smaller than the reference voltage VREF. That is, the voltage dividing resistors 31 and 3
As the voltage at the connection point 3 becomes smaller than the reference voltage VREF, the level of the voltage signal output from the error amplifier 35 decreases, and the level of the triangular wave output from the triangular wave generation circuit 37 changes. This is because the time required for the above becomes longer.

By the operation of the drive signal output circuit 30, the duty ratio of the drive signal output to the switching drive circuit 16 (specifically, the bases of the transistors 17, 25, and 27) changes the duty ratio of the operation signal supplied to the CPU. The voltage VO (voltage at the power output terminal JOUT) is
Feedback control is performed so that the set voltage is determined by the resistance ratio of 1, 33 and the value of the reference voltage VREF.

Next, in the switching drive circuit 16, the PWM comparator 39 of the drive signal output circuit 30 outputs a low signal.
When the level drive signal is output, the PNP transistor 17 is turned on and the NPN transistors 25 and 27 are turned on.
Turns off and the gate of the P-channel MOSFET 7
Since it is connected to the battery voltage VB by the P transistor 17, the P-channel MOSFET 7 is turned off.

Conversely, the PWM of the drive signal output circuit 30
When a high-level drive signal is output from the comparator 39, the PNP transistor 17 is turned off and the NP
N transistors 25 and 27 are turned on. Therefore, the gate of the P-channel MOSFET 7 is connected to the ground voltage via the diode 21 and the NPN transistor 27, and is connected to the output terminal JPO of the negative side charge pump circuit 23 via the diode 19 and the NPN transistor 25. .

Here, when the battery voltage VB is equal to or higher than the above-described threshold voltage VTH (= 6 V) and the operation of the oscillation circuit 52 provided in the negative charge pump circuit 23 is stopped (that is, when the operation is stopped). When the operation of generating the negative voltage in the negative side charge pump circuit 23 is stopped), the diode 21 and the N
Current (charge) to ground voltage via PN transistor 27
Is pulled in, the P-channel MOSFET 7
The gate-source capacitance CGS is rapidly charged and the P-
The channel MOSFET 7 turns on.

On the other hand, when the battery voltage VB is lower than the threshold voltage VTH (= 6 V) and the oscillation circuit 52 provided in the negative charge pump circuit 23 is operating (ie, the negative charge pump circuit). 23) (when the negative voltage generation operation is being performed), as shown in FIG.
Until the gate voltage of the channel MOSFET 7 substantially reaches the forward drop voltage VF of the diode 21, the P-channel M
The charging current of the gate-source capacitance CGS flows from the gate of the OSFET 7 to the ground voltage via the diode 21 and the NPN transistor 27, the gate-source capacitance CGS is rapidly charged, and the gate voltage of the P-channel MOSFET 7 is reduced by the diode. After the voltage has reached the vicinity of the forward drop voltage VF of the N.21, the output terminal JPO of the negative side charge pump circuit 23 (from the gate of the P-channel MOSFET 7 via the diode 19 and the NPN transistor 25)
The charging current flows to the capacitor 56), and the gate-source capacitance CGS is further charged, and the gate voltage of the P-channel MOSFET 7 finally becomes [VF-
3V].

For this reason, according to the switching power supply circuit of this embodiment, the battery voltage VB is
Gate-source threshold voltage V at which FET 7 can be turned on
Even when the voltage becomes about 3 V, which is lower than GS (Pth) (= 5 V), the gate-source voltage of the P-channel MOSFET 7 is set to be equal to or higher than the gate-source threshold voltage VGS (Pth).
The channel MOSFET 7 can be reliably turned on, and can operate even when the power supply voltage is lower.

In particular, in the switching power supply circuit of the present embodiment, the P-channel MOSFET 7 is used as a power supply transistor.
Since it is only necessary to generate a negative voltage smaller than the gate-source threshold voltage VGS (Pth) of the FET 7, the capacitors C1 and C
2. Inverters INV1, INV2, and diode D
The number of connection stages of 1 and D2 can be reduced, and the increase in circuit size and cost can be minimized.

That is, as a power supply transistor,
If an N-channel MOSFET is used, the ON-state gate-source threshold voltage VGS (Nth) of the N-channel MOSFET is the same as that of the P-channel MOSFET 5.
Assuming that the voltage is V, if the operation is to be performed even when the battery voltage VB is 3 V, the battery voltage VB must be boosted by 5 V or more, and a voltage of 8 V or more must be applied to the gate of the N-channel MOSFET. On the other hand, in the switching power supply circuit of the present embodiment, the negative charge pump circuit 23 may generate a negative voltage of about −3 V with respect to the ground voltage, and the capacitor constituting the negative charge pump circuit 23 may be used. C1, C2, inverter INV1,
The number of connection stages between INV2 and the diodes D1 and D2 can be reduced.

Therefore, according to the switching power supply circuit of the present embodiment, it is possible to operate even when the battery voltage VB is lower, despite a small circuit configuration. Moreover,
In the switching power supply circuit of the present embodiment, the P-channel MOSFET
Since the NPN transistor 27 (second switching element) for connecting the gate of the gate 7 to the ground voltage according to the drive signal from the drive signal output circuit 30 is provided, the current drawing capability of the negative side charge pump circuit 23 (that is, Even if the capacity of the capacitor 56 and the absolute value of the negative voltage charged in the capacitor 56 are small, the turn-on time of the P-channel MOSFET 7 can be shortened.

That is, when the NPN transistor 27 is not provided, the gate of the P-channel MOSFET
Gate-source capacitance CGS to the output terminal JPO side of the negative charge pump circuit 23 via only the PN transistor 25
Of P-channel MOSFET
7 is turned on. In this case, since the current-drawing capability of the negative-side charge pump circuit 23 has a limit, the P-channel M
The gate voltage of the OSFET 7 cannot be reduced quickly, and the turn-on time of the P-channel MOSFET 7 becomes longer.

On the other hand, if an NPN transistor 27 with a common emitter is provided in parallel with the NPN transistor 25 as in the present embodiment, the P-channel M
Until the gate voltage of the OSFET 7 becomes substantially equal to the forward drop voltage VF of the diode 21, a P-channel MOSFET
7, the charging current of the gate-source capacitance CGS can flow from the gate of the gate 7 to the ground voltage to rapidly charge the gate-source capacitance CGS.
Of the P-channel MOSF
The turn-on time of ET7 can be shortened.

Further, in the switching power supply circuit of the present embodiment, when the battery voltage VB is equal to or higher than a predetermined threshold voltage VTH (= 6 V), the operation of the oscillation circuit 52 in the negative charge pump circuit 23 is reduced. I try to stop it. That is, when the battery voltage VB is
The gate-source threshold voltage VGS (Pt
h), if the P-channel MOSFET 7 can be turned on simply by connecting the gate of the P-channel MOSFET 7 to the ground voltage via the diode 21 and the NPN transistor 27, the negative side charge pump circuit 23 is unnecessary. Therefore, in such a case,
That is, the negative voltage generation operation of the negative side charge pump circuit 23 is stopped.

Therefore, for example, only when the starter motor operates when the engine of the vehicle is started and the battery voltage VB drops, the negative voltage generating operation of the negative side charge pump circuit 23 is performed, and the negative voltage is generated. It is possible to extremely reduce the possibility that radiated noise (radio noise or the like) to the outside is generated due to the oscillating signal to be generated.

Incidentally, in the switching power supply circuit of this embodiment, the gate of the P-channel MOSFET
In each path between the two NPN transistors 25 and 27,
The diodes 19 and 21 are provided in the forward direction, respectively.
For the following reasons. First, in the switching power supply circuit of the present embodiment, as described above, the switching drive circuit 1
6 and the drive signal output circuit 30
As shown in FIG. 5A showing the cross-sectional structure of the NPN transistor 27, the element region of the NPN transistor 27 has N as a collector (C).
^{In addition to the +} portion, the P ⁺ portion as the base (B), and the N ⁺ portion as the emitter (E), the NPN transistor 27 is short-circuited with the emitter (E) for the purpose of not lowering the switching speed of the NPN transistor 27. A P ⁺ portion (hereinafter referred to as a ground P ⁺ portion) G connected to the ground voltage is formed.

That is, the ground P⁺Set up part G
The original base (B) becomes the emitter.
And the original collector (C) becomes the base,
For P ⁺The PNP transistor whose part G is the collector is shaped
When the NPN transistor 27 is turned off.
Then, the charge stored in the original base (B) is
The emitter (original base) of the formed PNP transistor
) To collector (ground P connected to ground voltage)
⁺Section G), immediately pull out the original NPN transistor
27 turn-off time (from turning on to off)
Time is not longer.

However, when the ground P ⁺ portion G is provided, as shown in FIG. 5A and FIG.
A parasitic diode Dk2 is formed in the forward direction between the emitter (E) and the collector (C) of the transistor 27, causing the following problem. That is, as described above, when turning on the P-channel MOSFET 7, two NPNs are used.
Transistors 25 and 27 are both turned on, and P-channel MO
The gate voltage of the SFET 7 finally tries to become a negative voltage by the negative side charge pump circuit 23 through the NPN transistor 25. However, if the NPN transistor 27 has the parasitic diode Dk2, the gate voltage of the SFET 7 becomes NPN from the ground voltage.
Via the parasitic diode Dk2 of the transistor 27 and the NPN transistor 25, the negative side charge pump circuit 23
Sneak current flows to the output terminal JPO of the parasitic diode Dk2. As a result, the gate voltage of the P-channel MOSFET 7 becomes lower than the ground voltage by the forward drop voltage VF of the parasitic diode Dk2.
(Approximately 0.6 V), the voltage drops only to a lower voltage (-VF). Moreover, the output terminal JPO of the negative side charge pump circuit 23 from the ground voltage via the parasitic diode Dk2 and the NPN transistor 25 (specifically,
Since an extra current flows into the capacitor 56), the output capability of the negative-side charge pump circuit 23 for outputting a negative voltage is reduced.

Therefore, in the switching power supply circuit of this embodiment, the gate of the P-channel MOSFET 7 and the two N
By providing diodes 19 and 21 in the respective paths between the PN transistors 25 and 27 in the forward direction, when the two NPN transistors 25 and 27 are turned on, the parasitic diode Dk2 of the NPN transistor 27 changes from the ground voltage.
Further, it is possible to prevent the current from flowing to the output terminal JPO of the negative side charge pump circuit 23 via the NPN transistor 25 and to prevent the current from flowing, thereby solving the above problem.

However, such a sneak current can be basically prevented by providing only the diode 21.
When the diode 21 is provided only on the NPN transistor 27 side, when the two NPN transistors 25 and 27 are turned on to turn on the P-channel MOSFET 7, the NPN transistor 27 is affected by the forward drop voltage of the diode 21. P-channel MOSFET
There is a possibility that the charge cannot be sufficiently drawn from the gate of the gate 7.

Therefore, in this embodiment, the P-channel MO
By providing the diode 19 also in the path between the gate of the SFET 7 and the NPN transistor 25, the path on the NPN transistor 27 side and the path on the NPN transistor 25 side are balanced.

In the switching power supply circuit of this embodiment, two NPN transistors 25 and 27
By providing diodes 19 and 21, respectively, NP
Preventing the switching speed of the N transistor 27 from decreasing,
The effect that the turn-on time of the P-channel MOSFET 7 can be shortened even when the current-drawing ability of the negative-side charge pump circuit 23 is small is sufficiently compatible.

On the other hand, in the switching power supply circuit according to the present embodiment, the switching drive circuit 16 and the drive signal output circuit 30 are formed by an insulation separation process, and each element constituting the circuits 16 and 30 is made of an insulator. Will be separated. Therefore, as described with reference to FIG.
The parasitic diode Dk1 as in the case of using the junction isolation process
Are the capacitors C1 and C of the negative side charge pump circuit 23.
It is possible to obtain the negative side charge pump circuit 23 which can generate a desired negative voltage without being formed in the area 2 or 56. In addition, in each part other than the negative-side charge pump circuit 23, an unnecessary parasitic diode is not formed in a part where a negative voltage is to be generated, so that the switching power supply circuit can be surely formed into an IC.

Although the embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and it goes without saying that various forms can be adopted.
For example, the negative charge pump circuit 23 may be configured as shown in FIG. Negative side charge pump circuit 2 shown in FIG.
3, the oscillation circuit 52 and the inverters INV1, INV2, and INV2 are similar to the negative charge pump circuit 23 shown in FIG.
Capacitors C1, C2, 56 and diodes D1, D
2, 54, but the other parts are different.

That is, the negative-side charge pump circuit 23 shown in FIG. 6 includes a Zener diode 60 having a cathode connected to the battery voltage VB, and two Zener diodes 60 connected in series between the anode of the Zener diode 60 and the ground voltage. Resistance 6
NPN in which a base is connected to a connection point of the two resistors 61 and 62 and an emitter is connected to a ground voltage.
A transistor 63, a resistor 64 having one end connected to the battery voltage VB, another end connected to the collector of the NPN transistor, an NPN base connected to the collector of the NPN transistor 63, and an emitter connected to the ground voltage.
And a transistor 65. In the negative charge pump circuit 23, the plus power supply terminal of the oscillation circuit 52 is connected to the battery voltage VB, and the minus power supply terminal of the oscillation circuit 52 is connected to the collector of the NPN transistor 65.

The negative side charge pump circuit 2 shown in FIG.
3, when the battery voltage VB is equal to or higher than a predetermined threshold voltage VTH, a base current flows from the battery voltage VB to the base of the NPN transistor 63 via the Zener diode 60 and the resistor 61, 63 turns on and the NPN transistor 65
Is turned off, and as a result, the power supply to the oscillation circuit 52 is stopped. On the other hand, when the battery voltage VB is lower than the threshold voltage VTH, the NPN transistor 63
Is turned off, a base current flows from the battery voltage VB to the base of the NPN transistor 65 via the resistor 64, and the NPN transistor 65 is turned on to supply power to the oscillation circuit 52. Then, an oscillation signal is output from the oscillation circuit 52, and a negative voltage is generated just like the negative side charge pump circuit 23 shown in FIG.

Therefore, even if the negative side charge pump circuit 23 of FIG. 6 is used, the same effect as the switching power supply circuit of the above-described embodiment can be obtained. Further, in the switching power supply circuit of the above embodiment, the circuit portion of the switching drive circuit 16 and the drive signal output circuit 30 is formed into an IC by an insulation separation process.
The IC including the T7 and the flywheel diode 9 may be included.

On the other hand, in the switching power supply circuit of the above-described embodiment, for example, the NPN transistors 25 and 2
7 or an N-channel MOSFET,
A P-channel MOSFET may be used instead of the transistor 17. Further, the switching power supply circuit of the above embodiment is provided in an electronic control device for an automobile. However, the switching power supply circuit of the present embodiment is similarly applied to other electronic control devices in which a power supply voltage supplied from the outside fluctuates. Can be used.

Further, the power supply target is a CPU.
The present invention is not limited to this, and various circuits, such as other logic circuits, which operate by receiving a predetermined operating voltage can be considered. In the negative charge pump circuit 23 shown in FIG. 2, the threshold voltage VTH at which the operation starts is generated by the voltage dividing resistors 43 and 44, and the voltage dividing resistors 41, 42 and 43 and 44 and the comparator 45 are used. Although the charge pump operation is controlled by such a circuit, such a circuit may be substituted by, for example, a separately provided power supply voltage monitoring circuit.

The threshold voltage VTH is not limited to 6V, but may be, for example, about 8V.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a configuration of a switching power supply circuit according to an embodiment.

FIG. 2 is a circuit diagram illustrating a configuration of a negative-side charge pump circuit in FIG.

FIG. 3 is a cross-sectional view illustrating a structure of a capacitor formed in an insulation separation step.

FIG. 4 is an explanatory diagram illustrating an operation of an ON drive unit of the switching drive circuit.

FIG. 5 is an explanatory diagram illustrating an operation of a diode provided in an ON drive unit of the switching drive circuit.

FIG. 6 is a circuit diagram illustrating another configuration example of the negative-side charge pump circuit.

FIG. 7 is a cross-sectional view illustrating a structure of a capacitor formed in a junction separation step.

[Explanation of symbols]

JIN: Power input terminal, JOUT: Power output terminal, 3, 11
... coils, 5, 13, 56, C1, C2 ... capacitors
7 P-channel MOSFET, 9 Flywheel diode, 15 Smoothing circuit, 16 Switching drive circuit, 17 PNP transistor, 19, 21, 54, D
1, D2: diode, 23: negative charge pump circuit, 25, 27: NPN transistor, 30: drive signal output circuit, 35: error amplifier, 37: triangular wave generation circuit, 39: PWM comparator, 52: oscillation circuit, IN
V1, INV2: Inverter, JPO: Output terminal

Claims (6)

[Claims] A power supply transistor provided in series on a power supply path from a power supply voltage to a power supply target; a switching drive circuit for turning on / off the power supply transistor in response to a drive signal; A smoothing circuit provided in a path between a supply transistor and the power supply target, for smoothing an output voltage of the power supply transistor and supplying the output voltage to the power supply target; A drive signal output circuit that outputs the drive signal to the switching drive circuit so that a predetermined set voltage is obtained.The switching power supply circuit, wherein the power supply transistor has a source in the power supply path. A P-channel field-effect transistor connected to the power supply voltage side and having a drain connected to the smoothing circuit side; The switching drive circuit, as an ON drive unit for turning on the P-channel field-effect transistor, receives the power supply voltage, generates a negative voltage lower than a ground voltage, and outputs the negative voltage from an output terminal. A switching power supply circuit comprising: a charge pump circuit; and a switching element that connects a gate of the P-channel field-effect transistor to an output terminal of the negative-side charge pump circuit in response to the drive signal. 2. The switching power supply circuit according to claim 1, wherein the ON drive unit of the switching drive circuit connects a gate of the P-channel field-effect transistor to the ground voltage according to the drive signal. And a switching power supply circuit. 3. The switching power supply circuit according to claim 2, wherein a diode is provided in a forward direction between a gate of the P-channel field-effect transistor and the second switching element. A switching power supply circuit. 4. The switching power supply circuit according to claim 2, wherein said negative-side charge pump circuit generates said negative voltage when said power supply voltage is equal to or higher than a predetermined threshold voltage. A switching power supply circuit configured to stop operation. 5. The switching power supply circuit according to claim 1, wherein at least the negative-side charge pump circuit is formed on a semiconductor substrate as an IC, and the negative-side charge pump circuit includes A switching power supply circuit, wherein each of the constituent elements is separated by an insulator. 6. The switching power supply circuit according to claim 1, wherein at least the switching drive circuit and the drive signal output circuit are formed on a semiconductor substrate as an IC, and the switching drive circuit is provided. A switching power supply circuit, wherein each element constituting the circuit and the drive signal output circuit is separated by an insulator.